spec: drop anything older than Fedora 13

spec: drop anything older than Fedora 13

RHEL 5 is based on libvirt 0.8.2, as was Fedora 13.  RHEL 5 also
happens to be the oldest box that we actively support with a
buildbot, so it is time to clean up some crufty conditionals in
the spec file that no longer are necessary for modern Fedora.

Although it is probably okay to make further simplifications to
a newer minimum Fedora version, that can be done as a later patch.
This patch just focuses on cleaning any comparison of %{?fedora}
that will always be true or false once we assume a minimum of F13.

* libvirt.spec.in: Make with_audit default to on. Move other
conditionals to a single RHEL-5 block. Simplify any fedora
comparison older than 13.  Document our assumptions.

Signed-off-by: Eric Blake <eblake@redhat.com>